The present invention relates generally to stents which are implantable or deployable in a vessel or duct within the body of a patient to maintain the lumen of the duct or vessel open, and more particularly to improvements in stent structures, stenting procedures, and processes for making stents.
Stents are expandable vascular and endoluminal prostheses, usually employed to keep a particular site in the blood vessels open and unoccluded, especially in the coronary and femoral arteries, following treatment such as dilatation by balloon catheter angioplasty. But these devices are also quite useful in other applications as well, such as in other tracts internally of the human body where an aid is required at a target site to maintain the lumen open and unobstructed. Examples are the tracheo-bronchial system, the biliary hepatic system, the esophageal bowel system, and the urinary tract system. In general, a vascular stent in particular must be sufficiently dimensionally stable to keep the vessel and lumen open while resisting recoil of its elastic wall that naturally occurs when the site within the vessel or luminal structure has been subjected to outwardly directed forces that are necessary to expand the elastic fibers, compress fatty deposits on the wall, and/or to deploy the stent, and to prevent an acute closure following dissection of the vessel.
In practice thus far, two types of stents have predominated for clinical vascular applications. One type, exemplified by a stent available from Cordis Corporation, is fabricated from a round wire laid into a zig-zag form, described more fully in U.S. Pat. No. 4,580,568. The other type is referred to as the Palmaz-Schatz stent, manufactured from a longitudinal tubular element with a narrow central lumen, as described in European Patent EP 81-0221570. In the Palmaz-Schatz type, the longitudinal metal tubular element is open at both ends and has rectangularly shaped, longitudinally oriented openings in its wall that form a pattern of confluent intersecting struts.
These wire and tubular stents, as well as other types, are expanded to a larger diameter by application of radial forces directed outwardly from within the lumen of the stent by inflation of a precision expansion balloon of a catheter on which the stent is mounted. Typically, the balloon is coupled for inflation at the distal end of a catheter that may have several lumens, such as to permit blood perfusion, guide wire (rail) advancement, and other purposes, as well as for pressurizing (inflating) and de-pressurizing (deflating) the balloon.
In the case of a wire stent, such as that of the '568 patent, although the wire has a round cross section with relatively low risk of causing significant injury to the vessel wall, it makes only line contact with the wall at each wire segment. More importantly, this type of stent lacks the radial strength to resist forces of compression, and thus, is unable to withstand elastic recoil of the vessel wall following expansion thereof such as after balloon angioplasty. As a result, a wire stent may suffer a loss in lumen diameter in the vessel of up to about 30%. For example, the wall of a vessel whose lumen has been expanded by balloon inflation to a diameter of 4 millimeters (mm) can undergo recoil within days to a diameter of 3 mm, despite implantation of a wire stent.
The Palmaz-Schatz stent described in the EP 81-0221570 patent has a relatively stiff structure with good resistance to compression, and therefore, the dimensional stability to resist the vessel wall's elastic recoil and maintain lumen diameter. But it has the disadvantage of presenting greater risk of injury to the vessel wall or damage to its expansion balloon because of sharp edges. Clinical practice and animal experiments have demonstrated that the sharp edges of individual struts of this type of stent can cut into the medial and intimal layers of the vessel wall. With typical stent wall thickness in the range from 65 to 100 microns, its edges are almost knife-like. A deep cut in the vessel wall from such an edge during deployment of the stent can signal disaster. Even minor scraping or other abrasion of the vessel wall from these edges or protuberances provoke the healing phenomena of smooth muscle cell hyperplasia, which leads to restenosis. Recent clinical data has shown also that the restenosis rate with this type of stent is in the range of from 25% to 35%, which lessens its advantage of rigidity against radial compression.
Also, since the stent is crimped onto an uninflated balloon of a balloon catheter for subsequent deployment, any sharp edges are prone to puncture or rupture the balloon at that time or during stent advancement through the vessel or during deployment. If the balloon cannot be inflated to the size necessary to properly deploy the stent because the membrane has ruptured or is lealdng, and so leaves the stent either unopened or only partly opened, it may not be possible to retract the stent. In that case, the stent will remain in the blood vessel as an unuseful foreign body, incapable of maintaining an open lumen, and possibly to ultimately cause complete blockage of the vessel. If that were to occur in a coronary artery, it could lead to myocardial infarction, and potentially, death of the patient. In the case of loss of the stent in a femoral artery, the result could also be total obstruction and result in a significant compromise of leg blood circulation. We have observed clinical procedures where the balloon ruptured because of the stent's sharp edges when the inflation pressure exceeded three atmospheres.
Moreover, because the target site in the vessel is often deep within the vessel or body of the patient, with the necessity to advance the stent on its balloon catheter through a long, often tortuous path of normal or diseased vessel, the likelihood increases that sharp edges of the stent will cut into arterial tissue and provoke an acute closure, or compromise or prevent advancement of the stent to the target site. Here again, then, the stent must be adapted to enable it be advanced on the balloon catheter in relatively smooth, unimpeded fashion through the vessel lumen to the predetermined target site a which it is to be deployed.
It is also imperative that the stent will expand radially and reliably in a symmetric manner in response to the forces exerted on its interior surface by the inflating balloon. Several studies conducted by the applicants have shown that existing tube type stents all too often expand asymmetrically, despite application of homogeneous radial forces by the expansion balloon. We have found that a significant part of this problem is that a considerable initial force is required to induce primary bending of the struts (i.e., to overcome structural inertia), so as to displace them from initial positions primarily parallel to one another into a more net-like or rhombic position. This initial force is higher than that required for further expansion of the stent to a fully deployed position.
Typically, at least a few of the struts of the most popular existing tube type stents remain in their parallel or substantially parallel original positions during deployment of the stent, thereby forcing other struts to undergo overexpansion with ongoing inflation of the expansion balloon, which causes asymmetric opening of the stent. The result is that some of the through holes in wall of the stent grow much larger than others, which lends them to accommodate inward protrusion of the vessel wall, impeding blood flow and causing turbulent in the region of the target or implant site. Ironically, this tends to induce restenosis which the stent was implanted to prevent.
Several clinical investigations have focused on ways to assure symmetrical radial expansion of the stent. For example, reliable symmetric opening has been sought by use of a high pressure balloons, inflatable to pressures ranging from 12 to 20 atmospheres. But this has several drawbacks, such as requiring the use of a second balloon (the high pressure balloon) at the target site, and causing additional trauma to the vessel wall which may include tearing and dissectioning that induces restenosis. A study recently completed at our clinic has shown that late lumen loss is proportionately greater when higher inflation forces are used to deploy the stent. Moreover, high pressure balloons are expensive (present-day cost ranges from about $600 to $1,000 per balloon), and like others, are not reusable.
Restenosis can originate not only at openings within the stent but at either or both of its ends where a transition exists in the lumen, from a stiff metal prosthesis to a very elastic vessel wall. At the transition region, considerable mechanical bending force is applied to the vessel wall by the presence of the stent--prompting a vascular reaction that leads to restenosis.
Although wire type stents, being more flexible, are less likely to cause this problem, they have the aforementioned inability to withstand vessel recoil. Moreover, the inadequate support of the diseased vessel wall offered by wire stents has been shown by many dissections. Wire tends to provide line support, which is inferior to the support given by the thicker, rectangular shapes typically found in the tube type stents. Wire meshes, like asymmetrically expanded tube type stents, can have very large holes that encourage inward protrusion of the vessel wall, with the same undesirable results.
All too frequently, the bending that occurs during expansion of known tube type stents causes twisting or torquing of at least some of their elongate strut members. In some instances, the twisting is attributable to weakness in the structure at locations where the struts are connected by bridges or bars of thicker or thinner cross-section, or where other regions of non-uniform thickness exist. When the struts become twisted, the vessel wall will be engaged by the stent, at least partly by the edge of the thin-walled (e.g., 65-70 microns thick) tubing, instead of the wider (e.g., 140 microns) side of the strut, with the aforementioned results of tissue or balloon membrane damage.
Also, if the physician finds it necessary to insert a balloon-mounted stent through an already-deployed stent, the order of difficulty is considerably greater where the latter has a twisted structure since it creates a region of higher friction in the lumen. This increases the possibility that the edge of the stent being deployed will become hooked distally of the existing implanted stent. The problem can occur where the site of a dissection to be stented was inadequately identified by dye, so that incomplete coverage is afforded by the stent now implanted, leaving a distally unsupported dissected region. Within a short time, typically from 5 to 15 minutes, the unstable vessel wall condition and the need to place a second stent distally of the first will become visually apparent.
It is a principal aim of the present invention to provide a stent which is less likely than those of the prior art to injure the vessel or tract wall during advancement through the lumen thereof, or to rupture the membrane of the balloon during initial mounting or subsequently while advancing or inflating the balloon for deployment, which requires less force for expansion and which expands symmetrically, and provides better protection against restenosis.